I. Field of the Invention
The present invention relates to semiconductor lasers. More particularly, the present invention relates to a novel and improved surface emitting laser wherein injection is achieved transverse to the direction of light propagation.
II. Description of the Related Art
Conventional semiconductor lasers are configured for emitting light from an edge-cleaved facet of the device. Recently, research has been conducted on surface emitting lasers (SELs) which emit light from a top or bottom surface of the device. Surface emitting lasers have an inherently distinct advantage over conventional edge emitting lasers in that the surface emitting lasers are readily adaptable for coupling to other optical components.
Research into the area of surface emitting lasers has resulted in the development of devices of various structures for emitting light from either a top or bottom surface of the device. In several of the previously known devices, carrier injection is achieved transversed to the direction of light propagation. However, many of these devices configured for transverse injection have high threshold current levels at which lasing occurs. The existence of a high threshold current level in a device is typically the result of the structure lacking transverse confinement of electrons and photons. A device having a high threshold current level is a device which is highly inefficient. A device lacking adequate confinement of electrons and photons results in overheating of the device due to the excessive current needed to drive the device to lasing. Accordingly, in low power requirement applications where temperature considerations are important, such inefficient devices may not be utilized.
One example of a surface emitting laser is disclosed in the publication "Room-Temperature Pulsed Oscillation of GaAlAs/GaAs Surface Emitting Injection Laser," Iga et al, Applied Physics Letters, Vol. 45, No. 4, pgs. 348-350 (1984). This publication discloses a surface emitting laser in which the cavity mirrors are formed by metalization, or a layer of SiO.sub.2 with metalization on top of the SiO.sub.2 layer. Carrier injection is achieved parallel to the pn junction with injection on the p-side of the device through the p-side mirror metalization. Utilizing this structure, threshold currents as low as 6 miliamps have been achieved with acceptable lasing output. However, this structure uses the p-side mirror to also function as an injecting contact. In order for the p-side mirror to act as an injecting contact, the metalization must be annealed to form an alloyed contact. The annealing process can result in poor mirror performance, primarily due to the absorption/scattering in the thin alloyed region formed beneath the metal layer. Implementation of a non-alloyed injecting contact can result in adverse local heating in the contact region during operation of the laser. Local heating in the contact region can alloy the contact and produce a drop in mirror reflectivity. A drop in mirror reflectivity can result in unstable laser performance.
Another example of a surface emitting laser is disclosed in the publication "Surface Emitting Laser Diode with Vertical GaAs/GaAlAs Quarter-Wavelength Multilayers and Lateral Buried Heterostructure," Ogura et al, Applied Physics Letters, Vol. 51, No. 21, pgs. 1655-1657 (1987). This publication discloses a lateral buried heterostructure laser in which a vertical distributed feedback active region is comprised of a quarter-wavelength stack of AlGaAs and GaAs layers that is surrounded with n-type and p-type AlGaAs cladding layers. This particular structure does provide enhanced carrier confinement with a threshold current realized as low as 2 mA. However, this particular structure exhibits a broad lasing spectrum with satellite emissions. This particular structure is fabricated such that a diffusion process is performed after fabrication of the structure mirrors. As such, the diffusion process may adversely affect mirror performance.